1. Field of the Invention
Embodiments of the invention are generally in the field of fluid mechanics and, more particularly pertain to electro-osmotic, capillarity-based apparatus, methods, and applications thereof and, even more particularly to switchable, electro-osmotic, capillarity-based apparatus and methods, and applications in the areas of adhesion and force transduction.
2. Technical Background
United States Patent Application Publication No. US2008/0037931, the subject matter of which is incorporated herein by reference in its entirety, discloses the meanings of the terms ‘switching device,’ ‘switching systems,’ and ‘capillary’. The '931 publication discloses, among other things, a retention system for the adhesive retention and release of one or more objects. The system includes a plurality of passageways arranged, adjacent to one another, each having two or more openings, and a force application system operatively associated with each individual passageway. A liquid in each of the passageways, having a volume that exceeds an internal volume of the plurality of passageways, forms a liquid drop around each of the openings. The force application system applies a force on the liquid to control switching between the two or more switch positions. The liquid drops are connected to one another by the liquid in each of the plurality of passageways. Each of the liquid drops is adjustable between two or more sizes and each of the sizes and a location of each of the liquid drops defines one of two or more switch positions. The liquid in each of the droplets has a wetability relative to the surface of the object that accommodates the object being retained or released by the droplets. Devices that operate with liquid droplets typically suffer from ‘volume scavenging,’ i.e., one droplet robbing volume from one or more adjacent droplets resulting in non-uniform droplet volumes and/or a coalescence of two or more droplets.
Certain animals exhibit extraordinary adhesion in daily activities and employ a variety of strategies to do so. The gecko is a prominent example, whose nano-fibrillar contacts are thought to rely on dry adhesion via van der Waals forces.
Wet adhesion strategies are also evident in nature, either relying on protein-based glues or a fluid mechanics-based bond via viscosity or surface tension.
Combined strategies have also been proposed for man-made devices (see, e.g., Lee H, Lee B P, Messersmith P B, A reversible wet/dry adhesive inspired by mussels and geckos, Nature 448:338-341 ((2007)).
The embodied invention as disclosed and claimed herein below, drew inspiration from the leaf beetle, an insect that achieves adhesion forces (˜33 mN) exceeding 100 times its body-weight. This is accomplished through the parallel action of surface tension across many micron-sized droplet contacts as reported by Eisner T, Aneshansley D J (2000) Defense by foot adhesion in a beetle (Hemisphaerota cyanea), Proc Natl Acad Sci USA 97:6568-6573.
A liquid droplet caught between two glass slides pulls the slides together. The liquid surface tension σ acts along the perimeter of the wetted contact-areas to give a force≈σπε for a single contact, where ε is the contact diameter. In defending itself by adhesion, the beetle establishes a large number N of small contacts, each of wetted area Awet. The beetle ‘feet’ project a total net area (i.e., including dry area between contacts) Anet≈2 mm2, and can deploy N≈105 contacts of ε≈2 μm. The net perimeter force scales as Nσπε, consistent with the measured adhesion of the beetle. To emphasize the geometric advantage of packing perimeter into a fixed area, we introduce a contact packing density φ≡NAwet/Anet. Using φ to eliminate N yields the perimeter force as F≈Anet(φ/ε2)σε, showing that F∞1/ε for fixed Anet. This amplification of the perimeter force by 1/ε illustrates the great benefit of packing a large number of small contacts into a fixed net area.
Similarly remarkable to the beetle's strength of adhesion is its quick ability to switch this bond on and off. Each contact can be thought of as switchable, and the beetle reconfigures its array of 105 contacts in less than a second. The beetle thus demonstrates the functionality of large arrays of small-scale capillary contacts for switchable adhesion.
Conventional techniques to grab surfaces use a vacuum/suction strategy, which suffers an intrinsic limit of adhesion strength, one atmosphere (≈100 kPa), due to their principle of operation. Further disadvantages of a vacuum device are bulkiness and the high power required to initiate and sustain attachment. Alternate mechanisms for switchable adhesion that have been demonstrated, including control of surface chemistry by temperature or pH, result in transitions that can take from minutes to hours to realize.
In view of the aforementioned shortcomings and disadvantages with the state of the art, the inventors have recognized the benefits and advantages of droplet-based apparatus and methods for rapid and repeatable attachment/detachment to wood, brick, linoleum, plastics, metals, and other surfaces of various roughness, which are designed to minimize or eliminate volume scavenging effects. Potential applications of such technology include, for example, load-bearing “Post-it®”-like notes, wall-climbing with “spiderman”-type gloves, and others. Further benefits and advantages are contemplated by apparatus and methods that would provide control with a precision that enables grab-release waves to be propagated along an active joint between two surfaces, e.g., one flexible and the other rigid. Zipping and un-zipping of adhesive bonds against a flexible component opens the possibility of reconfiguring (morphing) objects to take different geometric shapes—all in real-time. Still further benefits and advantages could be realized by force transduction apparatus and methods capable of exerting a force on an adjacent surface, making possible applications such as a credit-card-form device that could, e.g., pry open a rock fissure.